Vehicles known from practice are equipped with at least one transmission device disposed between a drive device and an output drive, in order to obtain various operating ranges, such as a forward gear, a reverse gear, and trailing throttle operation and a traction mode. The input torque of the drive device is transmitted to the front axle in front-wheel drive vehicles, to the rear axle in rear-wheel drive vehicles, and to two or more driven vehicle axles with various degrees of distribution in all-wheel drive vehicles.
In all these vehicles, the transmission devices are each equipped with a main gearbox, which can be used to obtain various gear ratios. Such main gearboxes are usually in the form of a manual transmission, multistage automatic transmission, or continuously variable automatic transmission.
In order to allow the distribution of the torque output by the drive device among multiple drivable vehicle axles in the longitudinal direction of a vehicle, each of the transmission devices of the above-described main gearboxes features a downstream longitudinal transfer case in the power flow of a drive train, which may take the form of separate structural units, can be integrated into the main gearbox, or may take the form of “hang-on” solutions on the main gearbox.
Moreover, the input torque supplied to a drivable vehicle axle is distributed between the two drive wheels of this drive axle using a transverse transfer case, wherein the drive wheels of the drivable vehicle axle can be driven independently of one another at different speeds in accordance with the different distances to be covered by the left wheel and the right wheel. As a result, the input torque can be symmetrically distributed to the particular drive wheels of the drivable vehicle axle and, therefore, free of yaw moment.
These advantages are offset, however, by the disadvantage that the drive forces of the drive wheels of a vehicle axle, which are transferred to the roadway, are dependent upon the lower or lowest transmissible input torque of the particular drive wheel due to the compensatory effect of a transverse transfer case, which is in the form of a differential. If a drive wheel positioned e.g. on slippery ice spins, for example, the other drive wheel is not supplied with more torque than the spinning drive wheel, even if the other drive wheel is positioned on a non-slip surface. In this kind of driving situation, the vehicle is unable to move forward, which is disadvantageous due to the compensatory effect of a differential, which enables two output shafts of a differential to rotate at different speeds.
To overcome this disadvantage, practical applications are now based on limiting or preventing compensatory movement of the differential by implementing suitable measures if critical vehicle states exist. This is achieved, for instance, via a differential lock known per se, which can be activated manually or automatically using mechanical, magnetic, pneumatic, or hydraulic means, which limits compensatory movement between the drive wheels completely or partially by locking the differential.
The differential lock may take the form of a hydraulically actuatable shift element, via which torque present at an input shaft of the differential can be distributed between two output shafts of the differential—which are interacting with the drive wheels—in accordance with a degree of distribution required. Output torque that is generated at one output shaft can therefore differ from the output torque that is generated at the other output shaft. This degree of distribution can be changed according to the operating state corresponding to the particular driving situation depending on the transmission capability of the shift element.
Differential locks of transfer cases designed as hydraulically actuatable shift elements are usually operated in a state of permanent slip. A robust design is therefore required, and high requirements are placed on the actuating accuracy, the actuating dynamics, and the dynamics during disengagement of the shift element since these have a strong influence on the driving dynamics of a vehicle.